The present invention relates to a packed column for exchanging heat and/or mass between a descending liquid and a rising gas, of the type comprising at least one exchange section defined between an associated upper liquid distributor and a lower inlet/outlet for fluid or an associated lower liquid distributor, this exchange section comprising an upper portion and, below the latter, another portion. It is applicable in particular to air-distillation columns.
Here the term xe2x80x9cpackingxe2x80x9d refers to a device designed to promote intimate contact between a descending liquid and a rising gas, so that heat and/or mass exchange can take place.
The packed portions each consist of at least one module or xe2x80x9cpackxe2x80x9d of packing. These modules may be of the random type, that is to say each consisting of a volume of discrete particles, but are preferably of what is known as the structured type. In this category, the following especially appear:
(1) Cross-corrugated packings, generally consisting of corrugated strips comprising parallel alternating corrugations each one arranged in a vertical general plane and one against the others, the corrugations being oblique and descending in opposite directions from one strip to the next. The degree of perforation is typically about 10% for these cross-corrugated packings.
GB-A-1 004 046 discloses packings of the cross-corrugated type, and CA-A-1 095 827 proposes an improvement on this type of packing by adding a dense arrangement of small-diameter perforations in order to allow the liquid to pass right through the cross-corrugated strips. Such a packing is generally manufactured from a flat product, namely metal sheets in the form of strips. The strips are firstly folded (or bent) so as to form a sort of corrugated metal plate in a strip, the corrugations of which are oblique with respect to the axis of the strip. The folded strips are then cut into portions and stacked, alternately turning around one band in two, so as to form modules or xe2x80x9cpacksxe2x80x9d.
(2) Ventilated packings, each module of which comprises a plurality of deflectors which define a set of horizontal layers of fixed ventilators for agitating the rising gas.
These ventilated packings are structures with a three-dimensional effect with regard to the gas which rises in the column. They ensure, by making the gas undergo multiple rotation, considerable turbulence in this gas, and the resultant agitation improves the exchange of heat and/or mass with the descending liquid.
Examples of such packings are described in WO-A-86/06 296, in WO-A-90/10 497 and in EP-A-0845 293.
The aim of the invention is to reduce the cost of packed columns for exchanging heat and/or mass.
To this end, the subject of the invention is a packed column of the aforementioned type, characterized in that the upper packed portion of at least one exchange section comprises at least one packing module which has a number of transfer units less than that of at least one packing module of the other packed portion of the same exchange section.
As is well known in the art, the number of transfer units (NTU) in the vapour phase of a packing module is the ratio of the height of the module (HM) to the height of a transfer unit in the vapour phase (HTU).
The HTU is commonly used to characterize the intrinsic efficiency of a packing, it is connected to the height equivalent to a theoretical plate (HETP) for this same packing by the equation:   HETP  =      HTU    ⁢          xe2x80x83        ⁢                            mV          /          L                -        1                    ln        ⁡                  (                      mV            /            L                    )                    
where
L: molar liquid flow rate (mol/s)
V: molar vapour flow rate (mol/s)
m: slope of the equilibrium line in a graph showing the molar fraction in the vapour as a function of the molar fraction in the liquid.
The HTU, and therefore the NTU, of a packing module depend to some extent on the fluids distilled and on the distillation conditions. However, when the NTUs of two packing modules are compared in this text, it is to be understood on each occasion that these two modules are placed in identical distillation conditions. In addition, to simplify matters, in this text, the number of transfer units in vapour phase will be called the number of transfer units.
The packed column according to the invention may comprise one or more of the following characteristics, taken alone or in all the technically possible combinations:
the said module of the said upper portion differs from the said module of the said other portion of the same exchange section by at least one of the following factors:
its intrinsic density;
its structure;
its mean density over the transverse section of the column; and
its height;
the said factor is the mean density, and the said upper portion (5A) occupies only a fraction of the transverse section of the column, located under the jets of liquid coming from the said upper distributor;
the said packed portions are portions of structured packing;
the said upper packed portion is a portion of cross-corrugated packing, while the other packed portion is a portion of ventilated packing;
the said upper packed portion is a portion of cross-corrugated packing of relatively low density, while the other packed portion is a portion of cross-corrugated packing of higher density, the packing modules possibly being offset angularly, especially by 90xc2x0, one with respect to the others around the vertical central axis of the column;
the said upper packed portion consists of a single packing module;
the said single module has a height lower than that of the modules of the other portion of the same exchange section;
in each exchange section, the packing modules are stacked one on top of the other, directly or with the insertion of elements forming spacers;
the column is part of a double air-distillation column; and
the other portion is a standard portion.